The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device, in which stacked plate layers in a lead frame can be prevented from peeling in a wire bonding procedure so as to attain high reliability of the connection between a wire and the lead frame.
A recently developed lead frame includes stacked plate layers, which are formed by plating a plate-shaped body of copper (Cu) with nickel as underplate and plating the nickel-plated body with palladium (Pd) and further with gold (Au).
Now, a conventional semiconductor device utilizing the lead frame including such nickel/palladium/gold stacked plate layers will be described.
FIG. 9 is a sectional view for showing the structure of the conventional semiconductor device. As is shown in FIG. 9, the conventional semiconductor device comprises a lead frame 1 including a die pad portion 2, an inner lead portion 5 and an outer lead portion 8, a semiconductor chip 3 bonded and mounted onto the die pad portion 2 of the lead frame 1 with an adhesive agent such as silver paste, and a metal wire 6 for connecting an electrode 4 of the semiconductor chip 3 with a bonding area 5a of the inner lead portion 5 of the lead frame 1. The peripheral area of the semiconductor chip 3, namely, an area including the die pad portion 2, the inner lead portion 5, the semiconductor chip 3 and the metal wire 6, is sealed with a sealing resin 7, so that the outer lead portion 8 of the lead frame 1 projecting outward from the sealing resin 7 can be suitably connected with external equipment.
Next, a method of manufacturing the conventional semiconductor device will be described. FIGS. 10 through 13 are sectional views for showing the manufacturing procedures for the conventional semiconductor device.
First, as is shown in FIG. 10, the semiconductor chip 3 is bonded onto the die pad portion 2 of the lead frame 1 with the adhesive agent such as silver paste (which procedure is designated as a die bonding procedure).
Then, as is shown in FIG. 11, the electrode 4 of the semiconductor chip 3 mounted on the die pad portion 2 is electrically connected with the inner lead portion 5 of the lead frame 1 through the metal wire 6 (which procedure is designated as a wire bonding procedure). This procedure is generally carried out by a nail head bonding method when a gold wire is used as the metal wire. Specifically, a wire bonder having a bonding tool 20 designated as a capillary is used, so that the metal wire 6 with a ball-like tip is pressed against and connected with the electrode 4 of the semiconductor chip 3 in a first bonding procedure and the metal wire 6 is successively pressed against and connected with the bonding area 5a of the inner lead portion 5 in a second bonding procedure. Through these two bonding procedures, the electrode 4 of the semiconductor chip 3 is electrically connected with the bonding area 5a of the inner lead portion 5 through the metal wire 6.
In this wire bonding procedure for connecting the electrode 4 with the bonding area 5a, the first bonding procedure is carried out under conditions of a ultrasonic power of 55 mW and a pressing load of 60 g and the second bonding procedure is carried out under conditions of a ultrasonic power of 90 through 100 mW and a pressing load of 100 g. The metal wire 6 generally has a diameter of approximately 30 through 35 .mu.m.
After completing the wire bonding procedure, the area including the die pad portion 2, the inner lead portion 5, the semiconductor chip 3 and the metal wire 6 is sealed with the sealing resin 7 as is shown in FIG. 12. This procedure is carried out by transfer molding with the lead frame 1 mounting the semiconductor chip 3 placed in a mold.
Ultimately, as is shown in FIG. 13, the outer lead portion 8 projecting outward from the sealing resin 7 is shaped. Thus, the conventional semiconductor device is manufactured.
The semiconductor device manufactured through the conventional manufacturing procedures has, however, the following problem:
FIGS. 14(a) and 14(b) are enlarged sectional and perspective views, respectively, of the vicinity of the bonding area 5a of the inner lead portion 5 in the conventional semiconductor device. As is shown in FIG. 14(a), the lead frame 1 includes a nickel plate layer 10, a palladium plate layer 11 and a gold plate layer 12 successively stacked on a body 9 of copper. The nickel plate layer 10, the palladium plate layer 11 and the gold plate layer 12 have thicknesses of 0.5 .mu.m, 0.03 .mu.m and 0.002 .mu.m, respectively.
At this point, as is shown in FIGS. 14(a) and 14(b), in the bonding area 5a of the inner lead portion 5, a peeled area 13 where the gold plate layer 12 is peeled is found to have been formed in a half circular shape around a tip portion 6a of the metal wire 6 where the metal wire 6 is pressed and cut with the bonding tool 20. The peeled area 13 can reach not only the palladium plate layer 11 but also the nickel plate layer 10 and the body 9 underneath. Due to the peeled area 13, the nickel plate layer 10 can be corroded and the strength in connection with the wire can be degraded. Additionally, a peeled plate material can be disadvantageously adhered to the bonding tool 20 of the wire bonder, so that the ball-like tip of a subsequent wire can be deformed in the subsequent wire bonding procedure (first bonding procedure). Thus, the peeled plate material can cause a wire bonding defect.
As a result of pursuit for the mechanism of causing the peeled area 13 in the second bonding procedure, it is estimated that the plate layer is peeled because the second bonding procedure is carried out with applying a larger ultrasonic power than in the first bonding procedure. Specifically, the current wire bonding procedure is carried out indispensably with applying the ultrasonic waves, the load and heat for instant and firm wire connection, so as to secure a high speed (mass production) and reliability in the connection. Furthermore, the gold plate layer has a thickness as small as possible (approximately 0.002 .mu.m) for decrease in the manufacturing cost. When the electrode of the semiconductor chip is connected with the inner lead portion through the metal wire while applying the ultrasonic waves to the lead frame having such a structure by using the wire bonder, the ultrasonic vibration cannot be sufficiently absorbed within the thin gold plate layer. Therefore, a crack is formed in the boundary between the gold plate layer and the palladium plate layer, particularly the periphery of the area pressed with the bonding tool, so as to ultimately peel the plate layer.